If you were unlucky enough to be locked in an airtight room, you might think you'd eventually run out of oxygen, but that's actually not true. More serious would be the excess of carbon dioxide.
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While the average human exhales about a kilogram of CO2 per day, that number varies considerably depending on body size. With vigorous exercise, that number could be eight times greater.
Having just passed the 8 billion population mark, how does this affect global warming?
It means our combined CO2 exhaled is about 2.97 billion tonnes - a significant proportion of the estimated 40.6 billion tonnes that we will emit this year. At first glance, that's a worrying number given that the atmospheric concentration of CO2 is now the highest it's been for 400,000 years.
The good news, however, is that the CO2 we exhale is part of a closed loop - as we respire, we are cycling carbon that is already active. The carbon in our bodies comes from food rather than fossil sources such coal, gas or oil.
In a quirky twist to this story, our bodies are in fact tiny carbon sequestration stores.
A human comprises about 18 per cent carbon. Numbers vary, of course, but that equates to roughly 9.8kg of stored carbon per person. Where that goes depends on how your mortal remains are shuffled off.
An environmentally friendly shallow burial is preferable to the 56 cubic metres of natural gas burnt if you're cremated. With conventional burial you'll also be emitting methane plus the byproducts of construction materials such as concrete.
That raises the question of the methane you exude while you're still alive. You deliver methane into the air via your breath and via flatulence. Again, the numbers vary and one can imagine how this would be affected by a few nights on the curry. Surprisingly, you also emit a tiny amount through your skin.
Overall, however, the number is not significant except on the small scale such as a crowded lift.
The far greater emissions impact of a human then, is not from our body, but from how we live. It's well known that those of us in wealthy countries generate considerably more CO2 through our lifestyles and industrial activities.
Much less known, or even acknowledged, is the rapidly growing impact of humans and consumption.
Each additional human adds to the load on the planet, representing a far greater opportunity than does a circular economy or even renewable energy.
Yet, bar a few exceptions, this is almost universally ignored by environment groups.
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